Process and apparatus for grading and for coating with comminuted material



Sept. 7, 1943. N. E. OGLESBY 2,328,577

PROCESS AND APPARATUS FOR GRADING AND FOR COATING WITH COMMINUTEDMATERIAL Filed Jan, 12,1940 2 Sheets-Sheet 1 10L. 1 Z0 lfl/VENTGR A TTOR/V5 Y5 Sept. 7, 1943. N. E. OGLESBY 2,328,577

PROCESS AND APPARATUS FOR GRADING AND FOR COATING WITH COMMINUTEDMATERIAL Filed Jan. 12, 1940 2 Sheets-Sheet 2 F X X X X X TRA/mFoRME/Q Mm/aosmrr FOR m3 VA R/ABLAT mu, m as a:

32 VAR/ABLE a ex-"a AC. VARIABLE.

MOTOR 3 FRZQ UENC Y GENERATOR INVENTOR N/c/vo/aa E. Qg/esy ATTQRNE Y3Patented Sept. 7, 1943 PROCESS AND APPARATUS FOR GRADING AND FOR COATINGWITH COMMINUTED MATERIAL Nicholas E. Oglesby, Troy, N. Y., assignor toBehr-Manning Corporation, Troy, N. Y., a corporation of MassachusettsApplication January 12, 1940, Serial No. 313,609

12 Claims.

I This invention relates to the separation or grading of comminutedmaterial such as abrasive grains, flocks, and the like, and isparticularly concerned with improved electrostatic separation orelectrostatic grading methods. More speciflcally this invention alsorelates to coating all or part of the separated or graded comminutedmaterial onto an adheslvely coated foundation material.

It is an object of this invention to coat the finer and lesseffectiveabrading particles, from a mixture of abrasive particles, onthe bottom of a coated abrasive coating and to coat the coarser abrasiveparticles on the top 01' the coating. It is a further object of theinvention to use an abrasive grain which has been less accurately gradedthan has heretofore been considered necessary and to coat such anabrasive in such a way that the top or effectiveabrading grains will bemore uniform in grade than the average grade or size of all the grainscoated. Another object.

of the novel method of coating provided in this invention is to avoidcoating relatively coarse particles with which the mixture of abrasivegrains delivered to the coating machine may be conterminated, or to coata finer grade of abrasive than the average grade of abrasive deliveredto the coating means. A further object of this invention is theproduction of a relatively deep coating of abrasive grains in orientedrelation, with a non-loading or open spaced oriented grain effect at thetop of the coating. Still another object of this invention is theprovision of suitable apparatus for carrying out the novel processesdescribed. It is a further object of this invention to separate abrasivegrains of variable size into two or more fractions, one of which iscoarser than the other. In stating the foregoing objects of theinvention, abrasive grains are given as a preferred illustration of thematerial worked upon by the novel process and apparatus. It is to beunderstood that other comminuted material, such as flocks, may be coatedor handled in an analogous way, and that abrasive grains arerepresentative materials that may be processed in accordance with thisinvention.

The method and apparatus by which my invention is carried out will beexplained by reference to the accompanying drawings, in which:

Figure 1 is a side elevation of one form of apparatus which may be usedfor carrying out my invention;

Filgure la is a section on the line la-ia of Figure Figure 2 is a sideelevation showing a modifies tion of the apparatus used in Figure 1;

Figure 3 is a side elevation of a form of apparatus for carrying outanother modification of the invention;

Figure 4 shows a wiring diagram for obtaining the variable frequencyalternating current of high potential used in carrying out my invention.

Similar reference characters refer to similar parts throughout theseveral drawings.

Referring to Figures 1 and 2:

Numeral l is an adhesively coated web such as paper or cloth, orcombinations of paper and cloth. Numerals 2 represent suitable idlerrolls over which the web l is continuously drawn through theelectrostatic field, by means not shown. Numeral 3 represents anelectrode which may be a solid electrode or a bank of electrodes asillustrated in Figures 1 and 2. In the same way numerals 4, 6 and 7represent electrodes which may be solid or a bank of individualelectrodes, as illustrated in the figures. The electrodes may be made ofsuitable metal or other conducting material and they may be insulated ornot insulated, but I generally prefer to use insulated electrodes,phenol formaldehyde resin or paper impregnated with such resins forminga suitable insulating covering for the electrodes or banks ofelectrodes. Numeral 5 represents connections between the individualelectrodes forming a bank of electrodes, as for instance, electrode bank3 may contain six electrodes connected together as indicated by numeral5. Various sized electrodes may be used but an electrode having adimension of about 6 inches in the direction of travel of the web andsurrounded by A inch of insulation has been found generally satisfactoryfor my purpose. With each individual electrode having a conductingdimension of 6 inches in the direction of travel of the web and inch ofinsulation surrounding the conductor, the electrode bank 3 containingsix such electrodes would have a total dimension of 3 feet in thedirection of travel of web I. In the same way, electrode bank 4 wouldhave a total dimension in the direction of travel of the web I of 2 feet2 inches, but I may use greater or shorter dimensions of electrodes inthe direction of travel of the web, according to the result desired.Numeral 8 denotes comminuted by conventional means, not shown. Numerali4 is a conducting connection connecting electrode 3 or electrode bank 3to ground. Numeral i5 is a conducting connection connecting electrode orelectrode bank 4 to ground. Numeral i 6 is a conducting connectionconnecting electrode 6 or electrode bank 8 to a source of high potentialof variable frequency and alternating polarity. Numeral I7 is aconducting lead connecting electrode 1 or electrode bank I to a sourceof high potential of variable frequency and alternating polarity or to asource of unidirectional high potential (direct current). The order ofconnections of electrodes may sometimes be reversed in that conductinglead id may be connected with a source of high potential of variablefrequency, and connecting lead it may be grounded. Likewise, conductinglead it may sometimes be connected to a source of high potential ofsuitable characteristics, and connecting lead ll may be grounded. Theabrasive grains 8 are delivered from hopper ill by feed roll iii ontothe endless belt l8 which is moved continuously into the electrostaticfield set up between electrodes 3 and E3. The high potential alternatingcurrent applied to electrode 6 where electrode 3 is grounded or toelectrode 2 where electrode a is grounded is of a variable number ofcycles, depending upon the grit sizes and variation in grit sizes beingseparated and coated. The number of cycles is adjusted so as to liftonly the smaller grains to the adhesively coated web, the larger grainsbeing lifted a shorter distance and dropping and/or being pulled back tothe carrier belt upon the reversal of polarity. I have discovered that asuperior separation'of the smaller and larger grains can be accomplishedby adjusting the frequency in accordance with the size of grains that itis desired to coat in the field between electrodes 3 and 6. I generallyprefer, especially in the case of fine grits, to use a voltage sumcientto lift all of the grains from the belt in, as I believe eflectiveseparation is brought about because the smaller grains travel fasterwhen lifted from the belt and, with the correct electrode spacing, reachthe adhesively coated backing at a point of time at which the largergrains have traveled a shorter distance. By a proper selection offrequency the polarity of the field is reversed before the larger grainsreach the adhesively coated backing. As stated above, the small grainsare lifted to the adhesively coated web while the larger grains arelifted a shorter distance. The frequency of alterations of polarity ofthe field is so adjusted and controlled that the average displacement ofthe finer particles per electrical impulse is substantially greater thanthe average displacement of the larger particles. Of course, the averagedisplacement of the larger particles is less than the free spacedistance between the electrodes 3 and 6, and, in fact, less than thedistance between the felt I!) and the adhesively coated surface of webI. Irrespective of the mechanism it has been found possible to coat the.finer grains in oriented relationship and have the larger grains passout of the field between electrodes 3 and 6 on th carrier belt l0. Asthe belt moves forward it carries the larger grains which are not coatedin the field between electrodes 3 and 6 into the field betweenelectrodes 4 and 1. Electrode l is connected to a source of highpotential of variable frequency when electrode 4 is grounded orelectrode 4 is connected to a source of high potential of variablefrequency when electrode 7 is grounded. Iimpress upon electrode 1 orelectrode 4, as the case may be,

a lower frequency alternating current whereby, for a given electrodespacing, a longer time of travel is permitted so that larger particlesreach the adhesively coated backing than was the case between electrodes3 and 6. By suitably controlling the frequency of the field betweenelectrodes 4 and l I may control the size of particles coated in thisfield. Often, however, it is desired to coat all or a large portion ofthe particles remaining on the belt in the field between electrodes dand i. In this case I may use direct current instead of an alternatingcurrent in establishing this field or I may use an alternating currentof relatively low frequency, as for instance, a frequency of 10 to 30cycles or lower. A separation of the grain into larger and finerfractions may take place in the field between electrodes i3 and 6 or inboth fields. In either case the average size of the grain coa%d dependsupon the electrode spacing, the voltage, and the number of cycles.

The separation desired is not attained by the use of a simple pulsatingdirect current with a variable frequency of pulsation, since charges arenot lost by the particles and the electrodes with sumcicnt rapidity, anda particle once charged and set in motion between the electrodescontinues to travel even though the connection to the source of directhigh potential is interrupted. Where alternating current is used thefield is reversed, and a particle with a given charge and in motion isreversed in direction of travel with the reverse in polarity and Ibelieve this to be the reason why better separations are attained withalternating current. Instead of using simple alternating current, acommutator may be used to reverse the polarity of the electrodes wherethey are energized with direct current but I prefer to use alternatingcurrent as it is simpler and very effective for my purpose.

The voltage impressed upon an electrode, the spacing between theelectrodes and different banks of eectrodes, the length of theelectrodes along the direction of travel of the adhesively coated web,and the frequency of the alternating current used, are varied accordingto the grit size and the result that it is desired to attain.

In the prior art, abrasive grains have been coated to a sheet by passingthe adhesively coated backing through an electrostatic field adjacent anupper electrode with the adhesive side facing downwardly and passingabrasive grains into the electrostatic field, often by means of a beltmoved adjacent to the lower electrode in its passage through the field.In this and in other methods of the prior art, spacings, voltages andfrequency of the current, where alternating current has been used toenergize an electrode, have been such as are conducive to coating theadhesively coated web quickly and completely with a grade of abrasivegrains which at least approaches the grade of the abrasive graindelivered to the electrostatic field. In actual practice, however, withdirect current, either uninterrupted or interrupted, or with alternatingcurrent of relatively low frequency and an air gap which is oftenrelatively small, the result has been that all of the abrasive grainsare propelled to the adhesively coated backing sheet. Some grains areretained by the adhesives, others immediately return to the carrier beltand are again propelled to the adhesively coated backing and thisprocess is continued as the web travels forward through the fieldbetween the electrodes. The weight of abrasive coated graduallyincreases due to repeated bombardments as the web passes through thefield. I! the energizing current furnished to the electrode is cut oilsuddenly, the pieces of backing within and corresponding to the lengthof the field at the time the current is broken may be sampled andanalyzed at any desired point within the coating field. It will be foundthat the grade of abrasive being coated as the web enters the field isusually at least substantially as coarse as the abrasive delivered tothe field, and that the overall grade of abrasive decreasesprogressively as the weight of abrasive coated is built up from thebeginning of the electrodes to the end of the electrodes in thedirection of travel of the web. By computation of grades, it will befound that the last abrasive coated as the web leaves the field is veryfine as compared with the grade of abrasive delivered to the field. Iattribute this to the fact that as the abrasive weight coated builds upin the field the spaces between the grains become small and it is,therefore, easier to embed the fines between the grains already coatedthan the average size or coarser size of grains still remaining in theabrasive mixture. At any rate, in the prior art the result has been theembedding of fine grains between coarser grains toward the end of thecoating operation in the electrostatic field. It should be understoodthat the presence of such very fine and inefiective grains between themore effective abrading grains is what we term a fine top effect in thisapplication, a condition which is conducive to loading of the product bythe work removed in abrading and which interferes with the efficiency ofthe product. It should be appreciated that a product built up accordingto the present invention avoids the difilculties of the prior art inthat a substantial proportion of the fines content is coated onto theadhesiveiy coated backing at the beginning of the coating operation,thereby removing these fines from the abrasive mixture so that they willnot be embedded between the coarser and more effective grains in thelater stage of the Coating operation wherein I provide conditionsconducive to coating coarser grains.

As an example of the use of the invention as illustrated in Figure l, Imay consider a coating carried out with grit No. 280 silicon carbideabrasive. The actual grading of the abrasive of this fine number iscommonly determined by sedimentation. From the sedimentation results, anaccumulation curve is plotted which shows the diameter of the grain interms of microns versus the percentage of grain. It should beappreciated that grains of this fine size and finer sizes, and sometimessomewhat coarser sizes, are graded by air or water classificationmethods. The grading' set forth, as determined by microns, is astatistical value, and the cleanliness of the grade can be determined byplotting a distribution curve. A clean grade is one having a highpercentage at or near the nominal size and relatively free fromsubstantially coarser or finer grades of grain. Where I speak of a flourgrade being finer than another flour grade I mean that the average grainsize of the finer grade is finer. Where I speak of a cleanly, well,evenly or uniformly graded material I mean that the material has arelatively steep distribution curve, shows a relatively high percentageof grain at or near the nominal size and is relatively free from grainsmuch finer or much coarser than the nominal size. At the 50.percentpoint on the accumulation curve, the standard diameter for grit No. 280silicon carbide is 44 microns. The abrasive delivered from the hopper 8oi. Figure 1 in this coating had a diameter at the 50 percent point onthe accumulation curve of 47 microns. This abrasive when coated by theregular method of upward propulsion, (i. e., electrostatic movement ofparticles counter-to-gravity) which does not involve the inventiondisclosed in this application to produce a coated abrasive article withan abrasive weight of 4.04 pounds per sandpaper ream, showed an overallgrading at the 50 percent point on the accumulation curve of 44.5microns. In the coating by the regular conventional method, the abrasivethat first adhered to the sheet was coarser than that which lateradhered to the sheet in the travel of the adhesively coated web betweenthe electrodes. The abrasive which was coated onto the adhesively coatedbacking just before the coated web emerged from the electrodes was veryfine, as was determined by stopping the machine and analyzing the gradeof the abrasive at several points along the line of travel of the webthrough the field. For both the conventional coating and the novelmethod herein described, 40 pound rope paper was used as the web to becoated and the adhesive coated on the backing by conventional means tohold the grain consisted of a solution of an, oil-modified alkyd resincomposition in high flash naph the, the solution containing about 65percent of nonvolatile resinous material. In the control experiment thecoating electrodes were energized with an alternating current of 25gcycles stepped up to 40,000 volts and the air gap between the beltcarrying the abrasive and the upper electrodes was uniformly inch.

In coating according to the present invention the apparatus shown inFigure 1 was used. The air gap between the upper bank of electrodes 3,which was grounded, and the belt l0 carrying the abrasive, was uniformly1% inches. The electrode bank 6 was energized by 60 cycle alternatingcurrent stepped up to 31,000 volts. It was desired to coat only a smallportion, and the finer portion, of the abrasive grain of grit No. 280 onthe belt ill in the field set up between. elec trodes 3 and 6. Theabrasive weight attained in the field between electrodes 3 and 6 was1.23 pounds per sandpaper ream and the grading of the abrasive coated atthe b0 percent point on the accumulation curve was 37.5 microns. Theadhesively coated web was 36 /2inches wide and was moved through thefield, adhesive side facing downwardly, at a speed of 102 feet perminute. The feed roll H delivered to the carrier belt to, in acontinuous manner, 5.4 pounds of abrasive per minute. The belt It! wasmoved through the field at a rate of 7 feet per minute. The layer ofabrasive on belt E0 was 38 inches wide. The coarser abrasive not coatedin the field between electrodes 3 and 6 was allowed to dump from thebelt at the point of roll I3 and was collected for re-use. In thecoating operation described the coated abrasive formed was finer ingrade and more uniform in grade than the abrasive fed to the belt l0.Also, the grains were coated in oriented relation.

In the form of apparatus shown in Figure 1, there is for a singlesetting, a constant air gap at all points between electrode bank 3 andthe carrier belt Hi. In practicing the invention with this form ofapparatus, the same or a diiferent spacing or air gap may be providedbetween electrodes 4 and I. In the form of apparatus shown in Figure 2there is, as shown in the figure, a gradual decrease in the air gap fromthe beginning of electrodes 3 and 6 to the end of electrodes 4 and i.For certain purposes there is an advan tags in the form of apparatusshown in Figure 2, in that a wider gap, conducive to good separation,may be used for the first bank of electrodes and a narrower gap,conducive to complete coating of all the grain, may be provided in asecond bank of electrodes, 4 and l. A variety of arrangemerits willoccur to those skilled in the art. The distance between the electrodebanks 3 and and electrode banks 4 and l is not critical so long as thetwo banks of electrodes are not so 'ciose to each other than there is aninterference between the two fields. In general I find a spacing ofabout 7 inches very satisfactory.

As a further illustration oi the use of my invention I may describe thecoating of grade No. hill-A waterproof sandpaper. In this instance theform of apparatus shown in Figure 2 was used. The air gap betweenelectrode bank and the abrasive-carrying belt H0 at the point where thebelt first enters the field was 1% inches. The spacing between the twopairs of electrodes was 7 inches. The electrode banks 3 and 6 were madeup of six electrodes with a dimension in the direction of travel of theadhesively coated web of 6 inches each (including insulation). Thesecond bank of electrodes was made up of only one electrode with adimension of 6 /2 inches (including insulation) in the direction oftravel of the adhesively coated web. The air gap at the end of the inch.As an adhesive, a 71 percent solution of oil-modified alkyd resincomposition dissolved in high flash naphtha was coated onto the web byconventional means, not shown. The web was 40 pound rope paperpreviously treated by conventional means to fill the pores andwaterproof the web. The web was 36 inches wide. A layer of No. 240silicon carbide abrasive, 38 inches wide, was fed onto abrasive-carrierbelt ill at the rate of 0.95 pounds per minute from hopper 9 by feedroll I l as shownin Figure 2. In the coating operation the web I wasmoved through the field with the adhesively coated side facingdownwardly, by conventional means, not shown, at a rate of 102 feet perminute. The abrasive-carrier belt i0 moved at a speed of 7% feet perminute. The abrasive grain fed had a grading of 59.5 microns at the 50percent point on the accumulation curve. Electrode bank 6 was energizedby an alternating current of 60 cycles at 24,000 volts. Electrode 1 wasenergized by an alternating current of cycles at 40,000 volts. Betweenthe electrodes 3 and 6 an abrasive weight of 2.35 pounds per sandpaperream was coated. At the 50 percent point on the accumulation curve theabrasive so coated graded 46 microns. Between electrodes 4 and 1, 3.25pounds per sandpaper ream of abrasive was coated, giving a total weightof coated abrasive of 5.6 pounds. The grading for the total abrasiveweight applied was, at the 50 percent point on the accumulation curve,56.5 microns. The abrasive applied in the first bank part of theabrasive, coated between electrodes 4 and 1, was much coarser than thatcoated between electrodes 3 and 6. In coating with both electrode banksa total of 76 percent of the abrasive delivered to the carrier belt IDwas coated and 24 percent of the abrasive fed was dropped off the beltand recovered for re-use as the belt electrodes 4 and 'i where thecarrier .belt it] emerges from the field was passed over roll 83. The 24percent of grain recovered at roll l3 was suitable for use in making acoarser grade of coated abrasive. The abrasively coated web I was passedby conventional from the abrasive-applying to a conventional drying ofthe use of mV coating of grade is a grade controlled by screens and notby sedimentation. In this instance the form of apparatus shown in Figure2 was used. The air gap between the electrode bank 3 and theabrasivewhere the belt first 1%; inches. of electrodes was 7 oinches.The electrode banks 3 and 0 were madeup of six electrodes each with adimension in the direction of travel of the adhesively coated web of 6inches (including insulation). The second bank of electrodes, Q and ii,was made up of two electrodes with a dimension of 6 inches each(including emerges from the field was inch. As an adhesive a 74 percentsolution of oil-modified alkyd per minute from shown in Figure 2.

30,000 volts. Between the electrodes 3 and 6 a sand weight of pounds persandpaper ream 1 was, therefore, decidedly coarser and freer from fines,as could be readily detected in the appearance of the coated product ascompared with a product coated by conventional counter-to-gravityelectrostatic means with the same abrasive to the same abrasive weight.In coating with both pairs of electrode banks, a total of 82.5 percentof the abrasive delivered to the carrier belt II) was coated and 17.5percent of the abrasive fed was dropped oil! the belt and recovered forre-use as the belt passed over roll l3. The 17.5 percent of grainrecovered was suitable for use in making a coarser grade of coatedabrasive.

In a manner analogous to which No. 240-A waterproof sandpaper was madewith grade No. 240 silicon carbide, I will describe a coating in whichNo. 320-A waterproof sandpaper was made. In this case conditions weresimilar to the manufacture of No. 240-A waterproof sandpaper, exceptthat a current of 120 cycles at 29,400 volts was used to energizeelectrode bank 6 and a current of 25 cycles at 35,000 volts was used toenergize electrode bank I. The air gap between electrode bank 3, in thiscase made up of three electrodes with a dimention 6 /2 inches each, inthe direction of travel of web I, and carrier belt in at the point wherecarrier belt i enters the field was 1 /3 inches. The air gap between thecarrier belt Ill and the electrode bank 4 in this case made up of twoelectrodes having a dimension of 6 inches each in the direction oftravel of web I at the point where carrier belt l0 emerges from thefield, was /2 inch. As in previou instances, the abrasive coated on thebottom between electrodes 3 and 6 was much finer than the abrasivecoated between electrodes 4 and 1. and the top coating of abrasivegrains was very free from fines and therefore more effective as anabrading instrument. The coated grains were well oriented.

No specific rules can be given for electrode spacings, frequency, andvoltage, for every condition that will be encountered. In general,however, electrode spacings in excess of 2 or 3 inches arenot preferredsince excessive voltages would berequired and since, for some reasonunknown to me, electrostatic action is more practical at smallerelectrode spacings. In general with the finer grits and a well coveredcarrier belt 8. voltage should be used which is high enough to readilylift the abrasive grains from the carrier belt and which is not highenough to cause arcing between the electrodes. The frequency of thealternating current used to make the separation in the coating operationin the step where the finer abrasive is removed from the abrasivemixture fed to the field varies with the grit size and the electrodespacing. Generally speaking, the wider the air'gap the lower thefrequency that can be used to make the separation, and the finer thegrain the higher the frequency desired to make a satisfactoryseparation. I select a practical air gap spacing and voltage and thenuse that frequency most effective for obtaining the desired results ofselectively removing a finer grade of abrasive from the coarser andvariable grade delivered to the field. I then use conventional voltages,electrode spacings, or air gaps and a direct current, eitheruninterrupted or interrupted, or a suitable frequency of alternatingcurrent for coating additional abrasive as required for the resultsdesired in the second abrasive-applying step, 'where such second step isemployed, as for instance, between electrodes l and l of Figure 1 and 2.

Figure 3 illustrates another form of my invention .and differs from theforms shown in Figures 1 and 2 in that in this form of the apparatus thefiner abrasive removed from the abrasive mixture fed to the device shownin Figure 3 is not permanently coated onto a reinforcing backing but is,on the contrary, removed after separation and collected for subsequentuse as desired. In Figure 3, numerals 2a represent idler rolls overwhich an endless belt 26 is trained, belt 26 being made of paper, cloth,or other suitable material. Idler roll 2b is driven by conventionalmeans, not shown, at any desired speed. Numerals 3, 6, 6, 6, 9, H, H and"5 denote parts which may be identical with parts denoted by similarnumerals in Figures 1 and 2 and already described. Numeral Illarepresents an endless belt which may be made of cloth or other suitablematerial. The belt Illa is trained about idler rolls l2a and a powerdriven roll l3a which is driven by conventional means, not shown, at anydesired speed. Numeral l8 denotes a suitable receptacle in which theabrasive or other comminuted material dumped from belt Ilia at roll I30may be recov ered. Numeral l9 represents a receptable or chamber inwhich the abrasive or other comminuted material removed from belt 26 maybe recovered. Numeral 20 represents a heater which may be used toevaporate liquid applied to belt 26. Numeral 2| represents a rattler orbeater which is driven by conventional means, not shown, and aids inthe'removal of abrasive or other comminuted material from belt 26 sothat it may be recovered in receptacle I8. Numeral 22 designates arotating brush driven by conventional means. not shown, to remove theabrasive or other comminuted material, if required, and as desired, fromendless belt 26, so that it may be recovered in receptacle l9. Numerals23 denote convention-a1 coating rolls driven by conventional means, notshown, which may be used to apply a liquid coating material to theendless belt 26. Numeral 24 represents a coating trough in which theliquid to be coated to roll 26 is placed so as to contact lower coatingroll 23. Numeral 25 denotes the liquid which is placed in coating trough24. In some instances, especially in the case of very fine grits, waterwill be found effective. In other cases, a small quantity of a surfacetension-lowering agent, such as sulphonated vegetable oil, sulphonatedhigher alcohol or other suitable surface tensionlowering agent is addedto the water and used. In still other cases a small quantity of solubleand usually mild adhesive, such as starch in solution, is used. In lieuof water, other volatile liquids or solvents may be used. The form ofthe invention shown in Figure 3 is particularly useful as anabrasive-separating mechanism. Furthermore, it is particularly useful inseparating the so-called flour grades or grades within the range of No.240 and finer. The abrasive which it is desired to separate, which maybe a mixture of different sizes of silicon carbide, aluminum oxide, orother abrasive, preferably containing particles of the size normallyfound in grades 240 and/or finer, is delivered from hopper 9 by speedroll H onto carrier belt Illa which is driven by roll l3a at any desiredspeed, as for instance, '7 to 10 feet per minute, through theelectrostatic field set up between electrodes 3 and 6, the rate of feedbeing governed by the results that it is desired to attain, aspreviously described in instances where it was desired to permanentlycoat thefiner fracture of grain removed from the abrasive mixture. Theendless belt 26 is coated with water or other suitable material, usuallywith little or mild adhesive action in the liquid state, by coating roll23 dipping in the coating liquid 25 in .trough 214, as the endless belt26 is moved in the direction of the arrow around idler rolls 2a by powerdriven roll 212 at a suitable speed, as for instance, 85 to 125 feet,although other speeds will often be found satisfactory. Electrode bank 3or electrode bank 6 is grounded and the opposing electrode is energizedby an alternating current, the frequency of which is controlledaccording to the results desired. I have found a frequency of 60 cyclesquite effective for grit sizes of the order of No. 220, No. 240 and No.280, provided the correct voltage is used and the correct air gap isemployed. As a rule a suitable air gap for grades of this size will beabout 1 to 1 /2 inches. By this I mean the air gap between belt ltd inFigure 3 and electrode bank 3. The air gap may be the same at the pointwhere the belt Mia enters the field as where the belt leaves the fieldor the gap may be varied, that is, electrode banks 3 and 6 may beparallel or at a slight angle to each other in accordance with theresults desired. Likewise, I may vary the length of the electrodes inthe direction of travel of the endless web 26 and the endless belt lua.Generally speaking, the longer the electrodes the greater .the quantityof abrasive that will be removed from the mixture of abrasive or othercomminuted material delivered to belt i012. In operating with grades ofthis size range, I prefer to use a voltage that will lift all or most ofthe grains from the abrasivecarrying belt in thefield between electrodes3 and S.

Where it is desired to separate grain sizes containing a considerableproportion of grains of still smaller size, that is, sizes smaller thanthe range 220, 240 and 280, Iprefer to use still higher frequencies bothfor those forms of the invention in which the finer grains separated arepermanently coated as in Figures 1 and 2 and in those cases where thefiner grain is removed from the field and collected for a subsequent useas is illustrated in Figure 3. For instance, for separating the finergrains in a mixture of grains of the order of magnitude of sizes 320,360, 400 and still finer, I may use frequencies between 60 and 180cycles, as for instance, 120 cycles, as has already been used inillustrating the coating of grade 320 by means of the form of apparatusshown in Figure 2. Generally speaking, the conditions depend upon thenature and size of the particles in the mixture being separated, and fora given material substantially the same conditions will be found tooperate where the finer grain is permanently coated as in Figures 1 and2, and where the grain is only temporarily coated or otherwise removedfrom the field and collected for subsequent use as illustrated in Figure3.

In the coarser grades, separation into different sizes is usuallyreadily accomplished by means of screens but in some instances thepresent invention is useful in handling coarser comminuted material,such as the coarser grades of abrasive. Where a coarser abrasive mixtureis separated by means of the present invention, a lower frequency may beused on the separating electrodes as for instance, electrodes 3 and 6 inFigures 1, 2 and 3. It should be appreciated that the controlling factorin selecting the correct frequency is that I select a frequency whichpermits the lifting of the finer portion of the mixture to theadhesively coated backing counter-to-gravity or in a direction with a,component counter-togravity, an angle too vertical often being conduciveto ease of operation of the equipment, and a frequency which alsoinsures a reversal of polarity before the larger grains reach theadhesively coated backing, or other means of removing the finer materialfrom the field.-

In general the preferred operation of my process with fine gritspresupposes that a definite layer of fine grits will be fed into thefield on a carrier belt, as for instance, belt in, or belt 50a, and thatsubstantially all of the grains will be lifted from the belt during theseparating operation. Where relatively coarse material is being handled,it sometimes may'be preferred to coat the belt sparsely with grain andlift only the finer grains from the belt. This can be accom plished bycorrect adjustment of frequency, air gap and voltage. Considering solelythe voltage, it should be understood that the voltage required is notalways proportional to the grit sizes. For instance, there is a certaingrit size, for example, of silicon carbide, that requires a highervoltage than either a smaller or larger particle to insure lifting ofthe particle. Considering the coarse grits, for example, of siliconcarbide, that is, grits coarser than about 180, a higher voltage isrequired to lift the larger grains than the smaller grains in this samerange. However, it frequently happens, and in fact, it is generallytrue, that a higher voltage is required to lift grit 280, for example,silicon carbide, than is required to lift grit or 120. In other words,there is a minimum voltage requirement in sizes in the neighborhood ofgrit 180 and the voltage required to lift the particles rises on thecoarse side of this point as the grit size increases and rises on thefiner side of this point as the grit size decreases. This condition,however, does not nullify the basis of this invention which succeeds inseparating fine abrasives even though a higher voltage may be requiredto lift the smaller particles, since in this grit range all particlesare set in motion but the finer particles once in motion travel fasterafter leaving the abrasive carrier belt, for example, and the coarserparticles are prevented from reaching the adhesively coated web or othercollecting means by a reversal in polarity at the selected frequency.

Referring to Figure 4 which illustrates diagrammatically a set-up forobtaining any desired frequency of pulsation of alternating current andany desired potential from an electrode used in my coating process, 38is a variable speed motor connected through suitable connecting means 35to an alternating current variable frequency generator 32. Suitablemotors, connections and generators are well known in the art, Thealternating current generated in through conductors 34 and 35 to theprimary winding 33 of a transformer. In conductor 35 between the motorgenerator and the primary winding 33 of the transformer is located arheostat 36 for varying the voltage. The transformer may be of anydesired type, but for purposes of illustration, I use a core typetransformer. At 3! is shown a ground for the secondary winding 38 of thetransformer, the secondary winding 38 of the transformer being connectedto a suitable conductor 39 which leads to a suitable terminal A: whichmay be one of the electrodes indicated as connected to a, high tensionset in Figures 1, 2 and 3 or may be a terminal which is in turnconnected to one of the aforesaid elecgenerator 32 is fed anelectrostatic field of alternating Polarity. D h

trodes. By such adjustments as are known in the art, the variable speedmotor 30, the generator 32, the transformer windings 33 and 33 and therheostat 36 may be controlled to obtain any desired frequency andpotential at the terminal Ai.

, Electricity of characteristics suitable for carrying out my inventionis readily supplied by apparatus of the type illustrated in Figure 4.This type of apparatus supplies electricity or characteristics welladapted for use in energizing electrode Ii of Figure 1, electrode 8 ofFigure 2 and electrode 6 of Figure 3. The same type of apparatus may beused to supply alternating current, usually of lower frequency, toelectrode I of Figure 1 or electrode 1 of Figure 2, where alternatingcurrent is used to energize electrode 1 of Figure 1 or of Figure 2. Itis to be understood that the electrostatic separations carried out asillustrated in Figure 1 between electrodes.

3 and 6, in Figure 2 between electrodes 3 and B and in Figure 3 betweenelectrodes 3 and i are analogous. The fundamental difference is thatwhere the apparatus is used for permanently coating to a reinforcingbacking the finer portion of comminuted material, separated from thecomminuted material of variable size, a relatively strong adhesive isapplied to the reinforcing backing 1 of Figures 1 and 2, whereas, whenit is desired to recover for subsequent use as such, the finer portionof comminuted material separated from the comminuted material ofvariable size, as between electrodes 3 and 6, in Figure 3, a liquid suchas water or a mild adhesive is used to temporarily anchor the finerportion of comminuted material to the endless belt or carrier 26 so thatthe finer portion of material may b collected as illustrated in Figure 3in receptacle I! for further use as such. Recovery of the finer portionof material removed-from the field by carrier 26 is facilitated by theheater of Figure 3 which evaporates some or all of the liquid used towet and adhere the grains temporarily to the carrier 26 or evaporatesthe volatile portion of the mild adhesive coated to carrier 26, so thatthe temporarily adhered grains may be more readily removed from the webby beater or rattler 2| and/ or revolving brush 22.

It will thus be seen that there has been provided by this invention,methods and apparatus suitable for coating a controlled grade ofcomminuted matter such as abrasive grains to an adhesively coatedbacking, for separating com minuted material such as abrasive grain intotwo or more portions, one of which is of finer grade than the otherportion, and for combining the steps of separating or grading comminutedma terial, such as abrasives, with the coating of the same to anadhesively coated backing, in a single operation. There has thus beenprovided by this invention, methods and apparatus in which the variousobjects hereinbefore set forth, together with many thorough practicaladvantages, are successfully achieved.- As various possible embodimentsmight be made of the mechanical features of the above invention, and asthe art herein described might be varied in various parts, all withoutdeparting from the scope of the invention, it is to be understood thatall matter hereinbefore set forth is to be interpreted as illustrativeand not in a limiting sense.

I claim:

1. A process of separating finer sized particles from comminutedmaterial having particles of different sizes therein, comprisingestablishin ing said comminuted material through said field, adjustingthe frequency of alternations of the polarity of said field to separatefrom the comminuted material in the field only the finer sized particlesof comminuted material, removing the finer sized particles so separatedfrom said field, and collecting separately from the finer sizedparticles so removed, the coarser particles of comminuted material.

2. A process of separating finer sized particles and coarser sizedparticles from comminuted material having particles of diiferent sizestherein,

comprising establishing an electrostatic field of alternating polarity,simultaneously passing through said field separate means for remov ingtherefrom the finer sized particles and the coarser sized particles,adjusting the frequency of alternationsof the polarity of said field toseparate from the comminuted material introduced into the field and todeposit upon the firstmentioned particle removing means only the finersized particles, thereby removing the finer sized particles from thefield, and separately removing on the second-mentioned particle removingmeans the coarser portion of said comminuted material.

3. In a process of coating a reinforcing backing with an abrasivematerial, the steps comprising passing an adhesively coated web throughan electrostatic field of alternating polarity of controlled frequencyadjacent an upper electrode of said field and with the adhesively coatedside facing downwardly, passing abrasive grains of variable size throughsaid electrostatic field adjacent the lower electrode, adjusting thefrequency of alternation of the polarity of said field so that a finerportion only of said abrasive ma terial is propelled to and embedded insaid adhesive in oriented relation whereby the abrasive coating appliedto said adhesively coated web is of a substantially finer grade than theabrasive grain delivered to said field and whereby a coarser abrasivegrain with less variation in size is prepared for other abrasive use.

4. A process of coating a reinforcing backing with an abrasive materialcomprising passing an adhesively coated web through an electrostaticfield of alternating polarity of controlled frequency adjacent an upperelectrode of said field and with the adhesively coated side facingdownwardly, passing abrasive grains of variabl size through saidelectrostatic field adjacent a lower electrode, adjusting the frequencyof alternation of the polarity of said field so that a partial coatingof a liner portion only of said abrasive material is propelled to andembedded in said adhesive in oriented relation, passing said web socoated through a second electrostatic field adjacent an upper electrodeof said second field, passing the abrasive grains not coated in thefirst electrostatic field into the second electrostatic field adjacentthe lower electrode, and creating electrostatic forces in said secondelec trostatic field to cause a coarser portion of abrasive to bepropelled to and embedded in said adhesive whereby the final compositeabrasive coating applied to said adhesively coated web is relativelyfree from fine grains that have been em bedded between the exposedportions of the coarser grains, and drying said coated web to set theadhesive and fix the abrasive grains to the backing.

5. A process of coating a reinforcing backing with abrasive materialcomprising passing an adhesively coated web through an electrostaticfield with the adhesively coated side facing abrasive supplied to saidfield, passing abrasive grains containing a substantial proportion ofgrains finer than the coarser portion of said grains through saidelectrostatic field and electrically propelling a partial coating ofonly a finer por- 2 tion of said abrasive onto said adhesively coatedbacking in said field, passing said adhesively coated backing sopartially coated with abrasive grains, while the adhesive is still in acondition to receive additional grains, through a second electrostaticfield, supplying to said second electrostatic field coarser abrasivegrains relatively free from grains of the size of grains coated in thefirst electrostatic field, and electrically applying an additionalportion of a coarser size of abrasive to said previously partiallycoated backing, in said second electrostatic field, and setting theadhesive on the web so coated to hold the abrasive grains in place.

6. A combined process of grading abrasives and coating abrasives onto areinforcing backing comprising passing an adhesively coated web throughan electrostatic field with the adhesively coated side facing downwardlyand adjacent an upper electrode, passing abrasive grains of suitable butvariable size through said field, adjusting the electrode spacing, thevoltage and the frequency of the alternating current used to energize anelectrode of said field in accordance with the grade and quantity ofabrasive that it is desired to coat on the backing in said electrostaticfield and thereby propelling a controlled coating of abrasive grainsinto said adhesive on said web, removing the web so coated with adhesiveand abrasive from said field and setting the adhesive, and collectingthe uncoated abrasive grains, which pass through said field and are of adifferent grade from those coated for subsequent abrasive use.

7. A process of coating abrasives onto a reinforcing backing, comprisingproviding at least one electrostatic field adapted to coat a finer gradeof abrasive and at least one electrostatic field adapted to coat acoarser grade of abrasive, passing an adhesively coated backing throughthe first-mentioned electrostatic field and then through thesecond-mentioned electrostatic field, passing abrasive grains havingfiner and coarser grades therein first through the first-mentioned fieldand then through the second-mentioned field and coating on theadhesively coated side of the backing by electrostatic means, thereby,at least one finer portion of abrasive grains and thereafter at" leastone coarser portion of abrasive grains whereby a composite abrasivecoating is built up on the backing which is relatively free from finergrains that have been embedded between the exposed portion of thecoarser grains, and then setting the adhesive to anchor the grains tothe backing.

8. A process of grading comminuted material such as abrasive grainshaving finer and coarser portions intermixed therein, comprisingestablishing between spaced electrodes, an electrostatic field ofalternating polarity of controlled frequency with the direction of forceof said field having a component parallel to the force of gravity,conducting through said field separate means for removing therefrom, atleast one fiiner portion of comminuted material introinto the field andat least one coarser graded portion of such comminuted material,adjusting the voltage, the electrode spacing and the frequency ofalternations of the polari y 01' said field in accordance with the gradeand quantity of material to be separated as the finer graded portion ofthe material, and thereby mov ing a'finer graded portion of the materialto the first-mentioned material-removing means for removal therewithfrom said field, and removing on the second-mentioned material-removingmeans the coarser graded portion of the material not so separated by theelectrostatic field, whereby the comminuted material fed to theelectrostatic field is separated into at least one finer graded portionand at least one coarser graded portion.

9. A process of manufacturing coated abrasives,

controlled frequency, said field having a component force of gravity,foundation material through the said electrostatic field adjacent anupper electrode of said field with the adhesively coated side facingdownwardly, feeding abrasive grains of variable size through said fieldbelow said adhesively coated foundation material and adjacent a lowerelectrode, adjusting the voltage, the electrode spacing and thefrequency of the alternation of the polarity of said field in accordancewith the grade and quantity of material that it is desired to depositelectrostatically upon said adhesive, and depositing thereby acontrolled weight of a controlled grade of abrasive onto said adhesivelycoated side of said foundation material, removing the residual uncoatedabrasive from said field and setting the adhesive to anchor the coatedabrasive grains to the said foundation material.

parallel to the coated backing, electrodes, each has a component offorce parallel to the force of gravity, means for passing an adhesivelycoated backing in sequence through the first and second pairs ofelectrodes adjacent an upper electrode in. each means for establishingan electrostatic field of alternating polarity between electrodes,means-for controlling the frequency of alternation of said field ofalternating polarity and whereby a predominantly finer grade ofcomminuted material may be propelled to the adhesively coated side ofsaid adhesively coated backing between the first coarser grade ofcomminuted material may be coated electrically, and means for feedingcomminuted material of variable size in sequence through the first pairof electrodes and then through the second pair of electrodes.

11. Apparatus for coating comminuted material, such as abrasive grainsonto an adhesively pairs of tlierebetween which has a component of forceforce of gravit the electrode spacings gradually decreasing from amaximum of electrodes to passing an adhesively coated trode in each pairof electrodes, means for establishing an electrostatic field ofalternating p0- larity between said first pair of electrodes, means forcontrolling the frequency of alternation of said field of alternatingpolarity and whereby a predominantly finer grade of comminuted materialmay be propelled to the adhesively coated side of said adhesively coatedbacking between the first pair of electrodes, means for establishing anelectrostatic field between the second pair of electrodes such that acoarser grade of comminuted material may be electrically coated, andmeans for feeding comminuted material of variable size in sequencethrough the first pair of electrodes and then through the second pair ofelectrodes.

12. A process of separating comminutedmaterial having particles ofdifierent sizes therein into finer and coarser portions comprisingestablishing an electrostatic field of alternating polarity betweenspaced electrodes having free space for motion of particles between theelectrodes, passing said comminuted material into said field,controlling the frequency of alternations of polarity of said field sothat the average displacement of the finer particles per electricalimpulse is substantially greater than the average displacement of thelarger particles per electrical impulse and so that the averagedisplacement of the larger particles per electrical impulse issubstantially less than the free space distance between the electrodes,and whereby there is created in the field and adjacent one electrode, azone containing a finer portion of comminuted material and. relativelyfree from the coarser portions of comminuted material, removing thefiner portion of comminuted material from this zone and collecting itfor a-subsequent use, and separately removing the coarser portion ofcomminuted material and collecting it for a separate subsequent use.

NICHOLAS E. OGLESBY.

